Small flow channels for liquids and gases are often required in micro-fluidic devices. These channels are too small to be created by conventional machining, and hence, techniques based on the bonding of multiple layers of metals have been developed. The layers of metal can be machined to provide channels, which, when bonded to each other form closed channels through which fluids can be moved.
Laminated multi-layer metal devices can be constructed by bonding metal layers using solid state bonding (e.g. diffusion bonding and thermal compression bonding), adhesive bonding, ultrasonic welding, etc. but the preferred method is diffusion bonding because the resulting device is a single monolithic block of material with internal features. This device, therefore, is much stronger than a device made by using, for example, adhesive bonding.
The lamination of multiple layers during the diffusion bonding process occurs in a low inert gas pressure or vacuum environment, at high metal layer temperatures, and using an applied force to create a substantial bonding pressure between the layers. Prior to the start of the process, the layers of material are stacked upon each other and pressure is provided by platens on the top and bottom of the stacked layers. The bonding must be done in a manner that prevents the layer from bonding or otherwise adhering to these lamination platens.
Currently, there are two prior art methods for preventing this undesirable adherence. First, the bonding layers may be fabricated from a material that does not adhere to the platen material at high temperatures. This method limits the types of materials that can be bonded, and hence, presents problems in some material systems.
The second method involves using a release agent between the platens and the outer surface of the layers. This release agent may be in the form of a fine powder or a sheet of separator material, or both. The release agent is chosen such that it does not adhere to either the platen or the layer materials at high temperatures under bonding pressure. For the bonding of some materials, it is difficult or expensive or not possible to make a separator sheet that is effective in preventing bonding where it is not wanted. In addition, for some materials, the use of separator sheets or release agents (which are often in the form of powders) leaves an imprint of the separator sheet surface or powder on the material surface, thereby degrading the surface finish.